In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least two layers: a photoconductive layer and an electrically conductive substrate. The electrostatic latent image can,be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
Among the many different kinds of photoconductive materials which have been employed in electrophotographic elements are phthalocyanine pigments such as titanyl phthalocyanine and titanyl tetrafluorophthalocyanine. These materials are generally insoluble; thus, photoconductive layers are usually produced from a liquid coating composition which includes a dispersion of the titanyl phthalocyanine pigment and a solvent solution of a polymeric binder. The titanyl phthalocyanine pigment is first prepared to convert it to a form, either crystalline or amorphous, that is highly photoconductive and capable of being sufficiently and stably dispersed in the coating composition to permit its being applied at a low enough concentration to form a very thin layer having high electrophotographic speed in the near infrared range.
A variety of methods have been used to produce suitable forms of titanyl phthalocyanine. Different methods have commonly produced titanyl phthalocyanines having differing crystallographic and electrophotographic characteristics. Many types of TiOPc and other phthalocyanines are discussed in Organic Photoreceptors for Imaging Systems, P. M. Borsenberger and D. S. Weiss, Marcel Dekkar, Inc., New York, pp. 338-391.
In one group of preparations, commonly referred to as "acid-pasting", crude titanyl phthalocyanine is dissolved in an acid solution, which is then diluted with non-solvent to precipitate the titanyl phthalocyanine product. In another group of preparations, the! crude titanyl phthalocyanine is milled, generally with particular milling media. Some preparations combine techniques or modify a previously prepared titanyl phthalocyanine.
U.S. Pat. Nos. 4,701,396; 5,153,094; 5,166,339; and 5,182,382 teach various acid pasting methods. U.S. Pat. No. 5,132,197, to Iuchi et al, teaches a method in which titanyl phthalocyanine was acid pasted, treated with methanol, and milled with ether, monoterpene hydrocarbon, or liquid paraffin. U.S. Pat. No. 5,206,359, to Mayo et al, teaches a process in which titanyl phthalocyanine produced by acid pasting is converted to type IV titanyl phthalocyanine from type X by treatment with halobenzene. U.S. Pat. No. 5,059,355, to Ono et al, teaches a process in which .alpha.- or .beta.-TiOPc was shaken with glass beads producing an amorphous material having no substantial peaks by X-ray diffraction. The amorphous material was stirred with heating in water and ortho-dichlorobenzene, Methanol was added after cooling. U.S. Pat. No. 5,194,354, to Takai et al, teaches that amorphous titanyl phthalocyanine prepared by dry pulverization or acid pasting can be converted, by stirring in methanol, to a low crystalline titanyl phthalocyanine, which was treated with methyl cellosolve or ethylene for a first polymorph; propylene glycol, 1,3-butanediol, or glycerine for a second; and aqueous mannitol solution for a third. U.S. Pat. Nos. 4,994,566 and 5,008,173, to Mimura et al, teach a process in which non-crystalline particles produced by: acid pasting or slurrying then mechanical grinding, mechanical grinding for a very long time, or sublimation; are treated with tetrahydrofuran. U.S. Pat. No. 5,055,368, to Nguyen et al, teaches a "salt-milling" procedure. U.S. Pat. Nos. 5,238,764 and 5,238,766, both to Molaire, teach that titanyl fluorophthalocyanine products of acid-pasting and salt-milling procedures, unlike unsubstituted titanyl phthalocyanine, Suffer a significant reduction in near infrared sensitivity when they are dispersed in a solvent such as methanol or tetrahydrofuran, which has a gamma.sub.c hydrogen bonding parameter value greater than 9.0. These patents further teach that this reduction in sensitivity can be preserved by first contacting the titanyl fluorophthalocyanine with a material having a gamma.sub.c hydrogen bonding parameter of less than 8.0.
Electrophotographic recording elements containing phthalocyanine pigments as charge-generation materials are useful in electrophotographic laser printers because they are capable of providing good photosensitivity in the near infrared region of the electromagnetic spectrum, that is in the range of 700-900 nm. In grey level digital electrophotography, and especially in laser imaging, it is very important to match the photoconductor sensitivity to the writing system. This is not a simple problem. Consideration must be given to such #actors as laser output energy, laser spot size, gray scale power levels, and temporal stability of the laser beam. The sensitivity of titanyl fluorophthalocyanine containing photoconductors can be adjusted. One way is by first selecting a charge generation material and them varying the thickness of the layer that contains that material. The photosensitivity is raised by increasing the thickness of the layer containing the charge generation material and lowered by reducing the thickness. This approach has limited utility, however, since it is only practical for a very narrow range of thicknesses. An excessively thin layer will not absorb enough light to permit charge erasure during an electrophotographic cycle. An excessively thick layer will not transport charges well. There is a further problem. This approach requires very close tolerances on the thickness of the layer containing the charge generating material. In manufacturing, such tolerances are likely to lead to greatly increased costs.
Another way of varying the sensitivity of titanyl fluorophthalocyanine containing photoconductors is by using a mixture of two different phthalocyanines. A number of references teach combining different titanyl phthalocyanines Different combinations of titanyl phthalocyanines have produced widely differing results.
U.S. Pat. No. 4,882,427, to Enokida et al, teaches that noncrystalline or pseudo-noncrystalline phthalocyanine products produced from various mixtures of crude phthalocyanines had sensitivities about the same as that of a noncrystalline titanyl phthalocyanine.
U.S. Pat. No. 5,112,711, to Nguyen et al, teaches an electrophotographic element having a combination of titanyl phthalocyanine and titanyl fluorophthalocyanine. In U.S. Pat. No. 5,112,711, a combination of titanyl phthalocyanine and titanyl fluorophthalocyanine provided a synergistic increase in photosensitivity, while combinations of titanyl phthalocyanine and chloro- or bromo-substituted titanyl phthalocyanine produced results in which the photosensitivity was nearer that of the least sensitive phthalocyanine.
U.S. Pat. No. 5,039,586, to Itami, teaches that a photoreceptor could be made having a mixture of such crystalline forms of titanylphthalocyanine, as .alpha.-TiOPc, .beta.-TiOPc, mixed .alpha.- and .beta.-, and amorphous TiOPc but does not indicate what electrophotographic characteristics would result.
Organic Photoreceptors for Imaging Systems, P. M. Borsenberger and D. S. Weiss, Marcel Dekkar, Inc., New York, at page 365, summarizes a number of reports of enhanced sensitivities for photoreceptors having charge generation layers containing mixtures of two phthalocyanines.
There is a continuing need for electrophotographic elements having various photosensitivities. It is highly desirable to provide improved electrophotographic elements including more than one type of titanyl fluorophthalocyanine and providing various photosensitivities.